The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a method and apparatus for cardiac MR imaging using an interleaved variable sampling-in-time scheme for data acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In imaging the heart, one has to contend with both respiratory motion and cardiac motion. The former being best controlled using a breath-held technique or some manner of respiratory compensation. Single-shot magnetic resonance imaging using Echo Planar Imaging (EPI) techniques are able to acquire an image in 50-100 msec, thereby eliminating cardiac motion artifacts, but result in low spatial resolution and image signal-to-noise ratio. Moreover, it is well known in the art that single-shot EPI acquisitions (including single shot spiral acquisitions) suffer from off-resonance effects which is manifested by either spatial distortion (with rectilinear read-out) or spatial blurring (with spiral acquisitions).
Spatial resolution and image signal-to-noise ratio (S/N) is restored by segmenting the acquisition over several cardiac cycles. In order to minimize the image blurring that results from cardiac motion over several cardiac cycles, the segmented acquisition approach gates data acquisition such that data for the desired image is acquired over a small temporal window within each cardiac cycle and gated such that the acquisition occurs at the same phase of the cardiac cycle over subsequent acquisitions. The segmentation of data acquisition over several cardiac cycles is often referred to as a segmented k-space acquisition.
Such acquisition techniques yield images with high image signal-to-noise ratio and high spatial resolution. By keeping the data acquisition window within each cardiac cycle short, cardiac motion blurring over this temporal window is minimized. However, a smaller acquisition window implies greater segmentation where all necessary data required to reconstruct an image is spread out over a larger number of cardiac cycles and increases the breath-hold period (scan time). With two-dimensional image acquisition using gated segmented k-space techniques, acquisition windows of between 50-100 msec have been used for scan times of between 12-20 seconds.
Obviously, with three-dimensional imaging, the amount of data is substantially increased due to the need to spatially encode for the third slice direction. Hence, for images at the same in-plane spatial resolution as in a two-dimensional acquisition, the total scan time is increased by a factor equal to the number of slice partitions in the three-dimensional volume. As a result, using the same acquisition parameters as the two-dimensional acquisition renders the scan time of a three-dimensional acquisition to exceed a single breath-hold time for a typical patient suffering from cardio-vascular disease.
In current three-dimensional cardiac imaging, due to the longer scan times, data acquisition is either respiratory-gated or breath-held using segmented echo planar imaging (EPI). If respiratory-gated, 3D CINE images are acquired over several minutes, and the quality of the data acquisition is dependent on the patient maintaining a relatively stable respiration pattern over a period of 6-10 minutes. Images acquired using such breath-held 3D acquisitions are often characterized by low spatial resolution with only a single phase of the cardiac cycle acquired. The acquisition period has been reported to be between 20 and 40 seconds. Volumetric imaging is accomplished by acquiring data over several different breath-hold periods and combining the data acquisitions. However, after reconstructing images with data acquired over different breath-hold periods, temporal and spatial discrepancies and inaccuracies can occur, resulting in images that are not well defined and/or blurred. Moreover, in order to attain these shorter scan times, the acquisition window in the current 3D acquisitions are often long. Thus, the need to accommodate a shorter breath-hold period leads to increased spatial blurring from cardiac motion as a direct consequence of a larger data acquisition window within each cardiac cycle.
In addition, respiratory-gated techniques using navigator echoes for monitoring the respiratory motion do not lend themselves to a multi-phase or CINE acquisition as a separate pulse sequence section must also be played out within each cardiac interval to interrogate the displacement of the diaphragm. Furthermore, the acquisition of data for the different cardiac phases may not necessarily be at the same respiratory phase, leading to mixed image quality. This is so because some phase images closer to the time when the navigator echo segment was executed have better image quality than that more distant in time.
Such conventional methods for assessment of myocardial viability, involves the identification of regions of delayed hyper-enhancement following administration of a contrast bolus using an inversion recovery segmented k-space fast gradient recalled echo (FGRE) pulse sequence. This technique requires multiple 2D sections, each of which is acquired in a separate breath-hold. In order to cover the entire heart in a short axis view, typically, between 8-10 sections are required. With each section acquired in a breath-hold of generally 12-20 seconds, total scan time is between 6-9 minutes. The additional time allows the patient to recover between breath-holds.
Repeated breath-holding, however, often results in rapid patient fatigue. Moreover, the length of time between the acquisition of the first and last sections can also lead to varying degrees of normal myocardial suppression and hyper-enhancement of infracted tissue in the resulting images. Repeated breath-holding may also lead to increased inconsistency of the breath-hold position. As a result, improper registration of the individual 2D sections may occur and introduce error in the measurement of the infarct size or volume.
One imaging technique that is directed to solving the aforementioned concerns utilizes a near-single breath-hold 3D cardiac data acquisition using variable sampling-in-time (VAST). In the proposed method, low spatial frequency data is sampled with a smaller temporal window (higher temporal resolution) than high spatial frequency data. Notwithstanding the advantages achieved by this method, transitions in k-space can adversely affect image quality. That is, this technique can result in a sharp, distinct discontinuity between the high spatial frequency views and the low spatial frequency views that may generate ghosting and/or artifacts in the resulting image.
It would therefore be advantageous to implement a technique for single breath-hold 3D imaging that eliminates such sharp transition and therefore provides images without the ghost artifacts associated with spatial frequency transition/discontinuity.